Rotational molding and casting techniques are quite highly developed and are well known within the plastic industry. The rotational molding of synthetic resins in liquid and powdered form is likewise well known.
Rotational molding, in general, comprises the steps of introducing into the mold, a predetermined quantity of a normally solid thermoplastic polymer composition in the form of fine particles capable of coalescing and melting at an elevated temperature. The shell mold is constructed from a thermally conductive metal and has an interior surface shaped to substantially correspond with the shape of the casting to be produced. The predetermined quantity of said polymer composition is such as to be sufficient to form a continuous layer having the desired thickness for the finished molded part. The next step involves closing the shell mold and rotating the closed mold about at least two different axes of rotation. The revolving shell mold is heated with a hot fluid, e.g. hot gas, that contacts the outer shell mold wall and has a temperature above that temperature which causes melting of the polymer composition. The heating of the revolving shell mold is continued until a coherent and substantially uniform layer of the melted polymer composition has formed on the interior shell mold surface. The revolving shell mold is cooled, and the article formed in the mold is discharged.
Attempts to obtain a good decorative appearance on rotationally molded hollow plastic articles by incorporating decorative particles in the plastisol mix or plastic mixture used in the molding process are generally not fully satisfactory because it was found that the particles agglomerated together near or at the surface and great difficulty has been experienced in obtaining a uniform distribution of the decorative particles throughout the article wall.